Measurement of interzonal heat and mass transfer by natural convection

Experiments to measure the interzonal heat and mass transfer were carried out in two full size adjoining rooms under two different conditions. Before starting the tests, one of the rooms was heated to an average temperature of 32°C, while the other room was cooled to an average temperature of 19°C. To start the first type of tests, the auxiliary heating and cooling were turned off and the door blocking the opening opened. For the second test, auxiliary cooling was turned off, while the auxiliary heat was left on in the heated room. Visual observations of the flow phenomenon were made. Velocity and temperature profiles of the air-flow through the opening, and air temperatures in the test rooms were measured. The experimental mass and heat flow rates were computed from the velocity and temperature data and compared with the values predicted by the existing algorithms based on the application of the Bernoulli's equation. The agreement between the measured and predicted values is better for a value of discharge coefficient C = 0.45 than it is for C = 0.611 (the theoretical value of C for a sharp edged orifice quoted in the literature).     

Experimental study of the natural convection flow around an array of heated horizontal cylinders

Natural convection heat transfer from an array of heated cylinders has recently attracted much attention. However, in almost all of the investigations, attention has been restricted to the influence of geometrical parameters on the overall heat transfer. The present work deals with an experimental study of natural convection flow from a vertical array of two heated horizontal cylinders. A detailed analysis of the local temperature and velocity fields is performed. The modifications of the average Nusselt number evolution compared with the single cylinder are explained in terms of velocity and temperature fields of the flow around the cylinders. Results that are obtained for a variety of combinations of spacings and Rayleigh numbers are compared with known numerical results. The comparison shows a satisfactory agreement.     

Experimental study of natural convection in PV roof solar collector

The aim of this paper is to propose the PV roof solar collector (PV-RSC) to investigate the natural convection heat transfer and estimated the convective heat transfer coefficient in the channel. The experimental set-up was composed of a PV panel on the upper layer and the lower layer is aluminum plate of the channel. The inclination angle and air gap of channel were fixed at 30° and 15 cm, respectively. The channel width is 0.7 m, and length is 1.2 m. The data analysis were confirmed the effect of radiative exchange influent to natural convection within the channel. On the basis of the experimental results, an empirical formula is found; the Nu as a function of Ra_s sin30, that is Nu_s = 0.3282 (Ra_s sin30)^0.2249. The correlation obtained to range 3 × 10⁸ < Ra_s sin30 < 7 × 10⁸. A comparison between PV-RSC and normal PV panel, it was confirmed that the PV-RSC could be generated electric power than that normal PV panel by about 30 W; and also the percentage of power generation increase was rising about 25% throughout the day.     

Experimental study of natural convection heat transfer in a semicircular enclosure

An experimental study of natural convection heat transfer in a differentially heated semicircular enclosure was carried out. The flat surface was heated and the radial surface was cooled isothermally. The effects of angle of enclosure inclination on the heat transfer across semicircular regions of several radii were measured for Rayleigh numbers Ra_R ranging from 6.72 × 10⁶ to 2.33 × 10⁸, using water as the working fluid. The angle of inclination varied from −90 degrees to 90 degrees with radii R of 50, 40, and 30 mm. The flow patterns were sketched from the results of a visualization experiment using aluminum powder. The temperature measurements in the enclosure were carried out using liquid crystals and thermocouples. The results indicate that different flow patterns were encountered as the angle of inclination varied, and the heat transfer rate was largely dependent on the flow pattern. In particular, enhanced heat transfer rates can be obtained when plume-like flow occurs along both hot and cold walls in the case of an upward-facing hot wall. Heat transfer for the inclined enclosure can be predicted using the equation for a vertical enclosure presented in this paper. © 1998 Scripta Technica, Inc. Heat Trans Jpn Res, 26(2): 131–142, 1997     

Experimental study on solar thermal conversion based on supercritical natural convection

In this paper, experimental investigation into the basic characteristics of solar thermal conversion using supercritical CO₂ natural convection are presented. Natural circulation of supercritical fluids can be easily induced and even a small change in temperature can result in large change in density close to the critical point. The supercritical experimental system carefully designed and operated in this study. It is found that an obvious and continuous long-time drop of solar radiation would not affect the CO₂ flow rate, temperature and pressure very much, if the solar radiation is in a relatively high-value level. This continuous drop can induce obvious drops in the CO₂ flow rate, temperature and pressure only when the solar radiation is in a low-value level. Furthermore, it is observed that a long-time drop and low-value in the solar radiation may make the flow rate temporarily become zero, which should be paid more attention in future system design and operation. The collecting efficiency increases with the comprehensive coefficient and this pattern is contrary to that of water based system. In addition, it is found that there exist optimal flow rate and CO₂ charge amount for system overall performance. This kind of solar thermal conversion has a higher collecting efficiency in spring and winter than summer and autumn; a better performance in cold and low-radiation region than hot and high-radiation region.     

Experimental study on natural convection of MWCNT-water nanofluids in a square enclosure

One of the methods to improve the efficiency of a wide range of energy systems is to enhance the performance of the heat transfer fluids. Nanofluids consisting of multi-walled carbon nanotubes in water base were studied experimentally in a square cavity with differentially heated side walls. The investigation was carried out for particle volume concentrations of 0 to 1% at Ra number 10⁸. The thermal conductivities and viscosities for the nanofluids were experimentally determined. However, it was observed that the available correlations from literature did not agree well with the experimental data. The nondimensional Nusselt number characterised the heat transfer performance. Thermal conductivity was measured for the range of volume concentration 0–1% and maximum enhancement of 6% was found to be at 1%. Viscosity was measured and observed to increase by 58% over the 0–1% particle volume concentration range tested. The experimental results on natural convection yielded a maximum enhancement in heat transfer performance of 45% at volume concentration of 0.1%. This research supports the idea that “for nanofluids with effective thermal conductivity greater than the thermal conductivity of the base fluid, there may exist an optimum concentration which maximizes the heat transfer.”     

Experimental study on transient natural convection in a cube enclosure with an isolated vertical cyclically heated plate

INTRODUCTIONNaturalconvectioninsideenclosuresisatopicofconsiderablecurrentinterestandimportance.Ofparticularinterestarethetransientcoolingproblemsinenclosureswithinternalisolatedheatedbodies,inwhichnaturalconvectionisoftenthedominantheattransfermecha...     

Experimental evaluation of natural convection solar air heaters

Four basic types of natural convection solar air heaters, in many geometries, were tested in a wide range of input solar energy values. Measured values of air flow rate, temperature increase and efficiency are reported. It is expected that the results and conclusions will be valuable in the design of equipment for drying, space heating and other purposes.     

Experimental determination of natural convection heat transfer coefficients in an attic shaped enclosure

Heat transfer by natural convection in triangular enclosures is an area of significant importance in applications such as the design of greenhouses, attics and solar water heaters. However, given its significance to these areas it has not been widely examined. In this study, the natural convection heat transfer coefficients for air in an attic shaped enclosure were determined for Grashof Numbers over the range of 10⁷ to 10⁹. It was found that the measured heat transfer coefficients could be predicted to within 5% by Ridouane and Campo's [E.H. Ridouane, A. Campo, Experimental-based correlations for the characterization of free convection of air inside isosceles triangular cavities with variable apex angles, Experimental Heat Transfer 18 (2) (2005) 81–86] equation (Eq. (1)) for natural convection in a triangular enclosure previously developed for Grashof Numbers in the range of 10⁵ to 10⁶.

equation(1)

Nu=0.286A−0.286Gr1/4.$Nu=0.286A^{-0.286}G{r}^{1/4}\text{.}$

Experimental study of natural convection in fluid-superposed porous layers heated locally from below

An experimental study of natural convection in fluid-superposed porous layers heated locally from below is reported. Measurements are made in a rectangular chamber with 3 mm DIA glass beads as the porous layer and distilled water as the saturating fluid. The effects of the heater-to-cavity length ratio and the porous layer-to-cavity height ratio on the overall heat transfer coefficients are reported. Average heat transfer coefficients over the heated surface increase with a decrease in porous layer-to-cavity height ratio, but no clear effect of heater-to-cavity length ratio is seen. Temperature profiles in the domain reveal a plume like flow with a single pair of circulating cells and evidence of convective motion inside the porous layer.     

Experimental and numerical study of the natural convection from a heated horizontal cylinder

Natural convection heat transfer from a horizontal cylinder is studied experimentally and numerically. Experimental study had taken place in different environmental temperature in a conditioned room which can be maintained at a stable required value and inside a sufficiently designed test cabin. The environmental and cylinder surface temperatures varied between 10 °C–40 °C and 20 °C–60 °C respectively. In the experimental study, two cylinders having different diameters of 4.8 mm–9.45 mm are used and constant heat flux was applied. On the basis of the experimental data, a correlation for the average Nusselt number over the cylinder is proposed in the range of 7.4 10¹ < Ra < 3.4 10³. The proposed correlation is compared with the well known correlations on natural convection heat transfer from a horizontal cylinder in the specified range of Rayleigh number, and it is shown that the results are in satisfactory agreement. The problem is also investigated numerically. The experimental data and the numerical results fall in ± 20% band. The numerical results obtained in this study are also compared with the results of Merkin. The characteristics of trend lines are similar.     

Natural convection in a vertical channel vented to the ambient through an aperture in the channel wall

Natural convection in a one-sided-heated, vertical channel in the presence of a vent opening in the unheated wall was studied both experimentally and numerically. The experiments were performed in water (Pr ≅ 5) for parametric variations of the interwall spacing, vent opening size, vent axial position, and wall-toambient temperature difference (Rayleigh number). For comparison purposes, baseline data were also obtained for unvented channels. The heat transfer data were supplemented by cross-channel temperature distribution measurements and by flow visualization performed using the thymol blue electrochemical technique. The numerical solutions provided information on both the local and average Nusselt numbers, cross-channel temperature profiles, and the rates of fluid flow through the opening at the bottom of the channel and through the vent. Both the experiments and the numerical solutions demonstrated that the average Nusselt numbers for the channel are insensitive to both the vent opening size and the vent axial position, for the range of operating conditions investigated. The numerical solutions showed that although the mass flow rate entering the channel through the opening at the bottom decreases in response to the presence of the vent, the total mass flow rate drawn into the channel is very little affected. The local Nusselt number distributions exhibited sharp, vent-related peaks. Excellent agreement prevailed between the experimentally and numerically determined cross-channel temperature profiles. For certain operating conditions, the flow visualization experiments revealed the existence of a recirculation zone inside the channel, adjacent to the lower edge of the vent opening.     

Experimental benchmark data for turbulent natural convection in an air filled square cavity

An experimental study of low-level turbulence natural convection in an air filled vertical square cavity was conducted. The cavity was 0.75 m high × 0.75 m wide × 1.5 m deep giving 2D flow. The hot and cold walls of the cavity were isothermal at 50 and 10 °C respectively giving a Rayleigh number of 1.58 × 10⁹. The local velocity and temperature were simultaneously measured at different locations in the cavity and both mean and fluctuation quantities are presented, i.e. ū, u^′_rms, v?, v^′_rms, , T^′_rms, , and . The local and average Nusselt numbers, the wall shear stress as well as the turbulent kinetic energy and the dissipation rate of the temperature variance are also presented. The experiments were conducted with very high accuracy and as such the results can form experimental benchmark data and will be useful for validation of computational fluid dynamics codes.     

Experimental study of natural frost formation over a horizontal tube with annular compact fins under natural convection

This paper presents experimental measurements of natural convection heat transfer and frost deposition over a horizontal fin‐tube. Measurements are made for a fin‐tube of diameter 25.4 mm, fin thickness 0.4 mm, fin center diameter 56 mm, and fin spacing 2 mm. For measurements the ambient air temperature and relative humidity are changed from 18 to 25°C and from 35% to 55%, respectively. The tube surface temperature is changed from –5 to –9 °C, and super cooling degrees of 7.5 to 24.5 °C. Results include a visualization of frost deposition growth, frost accumulation rate, and heat transfer rate with respect to time for each experiment. The results show that cold air starts from the upper point and moves downward and frost deposition starts on the fin tips, and grows with time both radially and angularly. Frost growth thickness changes significantly from top to bottom, where the boundary layers of both thermal and concentration increase at the bottom of the fin‐tube section without considerable separation. Frost growth only takes place on the fin's tip and it blocks the heat and mass transfer from the fin surfaces and the tube base which reduces convection and frost growth considerably. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20397     

Experimental study of flow and heat transfer characteristics of natural convection in an enclosure with horizontal parallel heated plates

We conducted an experimental investigation of natural convection in an enclosure containing horizontal parallel heated plates. Our experimental setup was as follows: Two heated plates were arranged vertically in an enclosure. The temperature of the ceiling was maintained at a constant low temperature, while the other sides satisfied thermal insulation boundary conditions. We examined flow characteristics in the enclosure and heat transfer around the horizontal heated plates. In the region between the cooled ceiling and the upper heated surface, vortex motion occurred when the accompanying flow from the lower region interfered with the ascending flow. Heat transfer around the heated surface was enhanced by this vortex motion. In other regions, the flow circulated along the heated surfaces in all cases. Generally speaking, for each flow pattern, the Nusselt number was proportional to some power of the modified Rayleigh number.     

Experimental study of natural convection heat transfer in a microencapsulated phase change material slurry

A new microencapsulated PCM (Phase Change Material) slurry (MEPCS) at high concentration (45% w/w) was developed based on microencapsulated Rubitherm RT6. Its heat storage and heat transfer characteristics have been experimentally investigated in order to assess its suitability for integration into a low temperature heat storage system for solar air conditioning applications. Differential scanning calorimetry tests have been conducted to evaluate the cold storage capacity and phase change temperature range. An experimental setup was built in order to quantify the natural convection heat transfer occurring from a vertical helically coiled tube immersed in the MEPCS. First, tests were carried out using water in order to obtain natural convection heat transfer correlations and then a comparison was made with the results obtained for the MEPCS. It was found that inside the phase change interval the values of the heat transfer coefficient for the MEPCS were significantly higher than for water, under identical temperature conditions.     

Experimental study of natural convection heat transfer of air in a cube below atmospheric pressure

Experimental study was carried out on the temperature profile of natural convection of air in a 50 mm cube with the temperature difference of 30 K between two opposing vertical walls. The objective is to investigate the essential aspect of heat transfer in the wide range of Rayleigh number (Ra) by lowering the pressure. The pressure was varied from 5.40 kPa (40.5 mmHg) to 99.99 kPa (750.0 mmHg). These correspond to Ra=1.04×10³ to 3.56 × 10⁵. The results show that the temperature distribution and Nusselt number approach those of conduction state as the pressure decreases.     

Experimental and numerical study of transient natural convection due to mass transfer in inclined enclosures

Both experiments and numerical work are performed to study transient natural convection flow and transport process due to mass transfer in the enclosures inclined at different angles. In the experiments, the enclosure is filled with aqueous solution containing CuSO₄ + H₂SO₄ where the flow structure can be visualized by both particle tracer and shadowgraph. Two opposed side walls of the enclosure are maintained at different concentrations which are made by passing current through the electrodes at the limiting condition. All the other side walls are made insulated and impermeable to the species transfer. Both the concentration distribution and its fluctuations are measured with non-intrusive optical method. During the experiments, the Rayleigh number ranges from 1.126 × 10⁸ to 1.157 × 10¹¹, the angles of inclination from 30 to 90 degree and the aspects ratio of the enclosure from 0.6 to 1. Comparison between the data and prediction is made and discussed at various conditions.     

An experimental study of natural convection in a differentially heated cavity through a 2D-PIV system

This paper provides an experimental analysis of the natural convection in a differentially heated H side square enclosure. The cavity is full of air and is heated by a hot strip with a height of H/2. The effect of the position of this source on the dynamic structures generated by the natural convection heat transfer was analyzed at the steady state and under laminar conditions. The experimental apparatus is a 2D-PIV system while the experimental data consist of vector maps, velocity maps and streamlines at different Rayleigh numbers. During the study the presence of two small vortexes was noted on the upper surface of the source. These are dependent both on the Rayleigh numbers and on the position of the strip.     

纵向涡强化竖直平板自然对流换热的实验研究

对纵向涡强化竖直平板自然对流换热进行了实验研究。结果表明，在一定的Rayleigh数范围内，直角三角翼纵向涡发生器的攻角、翼高、翼宽等几何参数是影响强化换热的主要因素。存在最佳攻角；宽高比一定时，翼高和翼宽的变化会影响换热的效果。发现在直角三角翼阵列中前排直角三角翼产生的纵向涡可以强化后排直角三角翼纵向涡的换热。将直角三角翼与矩形低肋换热表面的性能作了对比性实验，在其他条件相同的情况下，直角三角翼强化换热的效果优于矩形低肋。